(a) Field of the Invention
The present invention relates to a novel liquid-crystalline copolymer. More specifically, the present invention relates to a novel liquid-crystalline copolymer which exhibits ferroelectricity even at temperatures around room temperature, has a long helical pitch, and has such a high speed of response to external factors as to enable display of motion pictures. Such a liquid-crystalline copolymer is useful in optoelectronics fields as various kinds of optical elements, particularly, as those for display devices for desk calculators, clocks and watches, etc., and those for various electronic optical devices for electronic optical shutters, electronic optical diaphragms, optical modulators, optical-path transfer switches in optical communication systems, memories, liquid crystal printer heads, varifocal lenses, etc. In particular, the liquid-crystalline copolymer of the present invention has a high practicality in its use as the display elements for large display screens or curved display screens.
(b) Description of the Related Art
Display devices in which low molecular weight liquid crystals are used as the display element have been widely used for digital display of desk calculators, clocks and watches, etc. In these fields of utilization, the conventional low molecular weight liquid crystals are generally supported between a couple of glass substrates spaced from each other in microns. However, such an adjustment of the space has been practically impossible in production of large display screens or curved display screens. In order to solve the problem, some attempts have been made to develop polymeric liquid crystals so as to render moldability to the liquid crystals themselves. For example, in Japanese Patent Application Kokai Koho No. 55-21479, Japanese Patent Application Kokai Koho No. 63-99204, and EP-0184482 disclosed are various kinds of polyacrylate-type ferroelectric polymeric liquid crystals. Nevertheless, these conventional polymeric liquid crystals have hardly been satisfactory for practical use because of their high temperature ranges where ferroelectricity is exhibited. Further, these polymeric liquid crystals have deficiencies in that their speeds of response in the changes of their transmission intensity to the changes of external factors such as electric field are generally slow and sufficient bistability cannot be attained because of their short helical pitches.
Since ferroelectric liquid crystals exhibiting chiral smectic C phase (SmC* phase) have a layered structure and the director `n` of the molecules gradually rotates about the normal line of the layers with each layer, the directors `n` of the molecules on the whole construct a helical structure. The distance between the planes perpendicular to the helical axis required by the director to rotate for 360.degree. is called helical pitch. For instance, DOBAMBC has a helical pitch of from 2 to 3 .mu.m which, considering the size of the molecule, indicates that one pitch consists of molecule layers close to 1000. In comparison with the conventional nematic liquid crystals, ferroelectric liquid crystals are characteristically used as the materials of display devices because of their high speed response property and bistability (memory property). There exists a close connection between bistability and the helical pitch of ferroelectric liquid crystals, and, in order to obtain a display device having bistability, it is necessary to make the cell gap `d` smaller than the helical pitch `p` so that the ferroelectric liquid crystal operates at a state with its helix loosened. For this reason, there has been a demand for development of ferroelectric liquid crystal materials having long helical pitches.